Nine (9) human genetic neurodegenerative diseases, including Huntington's disease, are caused by the expansion of CAG repeat sequences in the coding regions of different proteins. These proteins differ in size, structure, sub-cellular localization, and, where known, function. Since their only common feature is the polyglutamine (polyGIn) sequence encoded by the CAG repeat, and since for all but 1 of these proteins the repeat length threshold for disease risk is in the same 30-40 length range, neurotoxicity is almost certainly triggered by some property of the expanded polyGIn sequence. This proposal builds on and continues the threads of the previous grant by investigating in more detail the biophysical properties of expanded polyGIn sequences, focusing in particular on the formation of amyloid-like aggregates. In recent work we showed that nucleation of polyGIn aggregation involves a highly unfavorable folding reaction within monomeric polyGIn; The first aim of this proposal is to investigate in greater detail the elongation and nucleation phases of the aggregation reaction, and their responses to changes in solution conditions. In very recent work we showed that the nucleation reaction, and hence the overall aggressiveness of aggregation, is greatly influenced by benign-repeat length polyGIn peptides in the environment; The second aim of this proposal is to investigate how local sequence context and mutational breaks in the polyGIn sequence, as found in Gin-rich sequences in the human genome, impact aggregation ability. In other recent work we showed that the molecular chaperone Hdj2, previously shown to protect organisms against polyGIn toxicity, inhibits the polyGIn aggregation process in vitro; The third aim of the new proposal is to determine structurally how Hdj2 recognizes and neutralizes aggregation-prone polyGIn sequences. These studies will continue our long-term goal of determining how polyGIn sequences form toxic aggregates, and how both sequence and cellular contexts modulate their intrinsic aggregation propensity, thus ultimately determining disease risk.